treat identical enum types as the same type

treat identical enum types as the same type

This is to avoid an unnecessary multiple-choice menu for an
expression involving an enumeral declared in two types, when
the second type is an identical copy of the first type. This
happens in the following situation:

       type Color is (Black, Red, Green, Blue, White);
       type RGB_Color is new Color range Red .. Blue;

In that case, an implict type is created, and is used as the base
type for type RGB_Color.  This base type is a copy of type Color.
We've added some extensive comments explaining the situation and
our approach further.

gdb/ChangeLog:

        * ada-lang.c (ada_identical_enum_types_p): New function.
        (symbols_are_identical_enums): New function.
        (remove_extra_symbols): Do nothing if NSYMS < 2.
        Use symbols_are_identical_enums.

gdb/testsuite/ChangeLog:

        * gdb.ada/same_enum: New testcase.